Gold (Au) nano shells are nanoparticles usually composed of a dielectric core, typically silica, coated with an ultrathin Au layer. These nanoparticles show interesting optical and chemical properties for the applications of surface-enhanced Raman spectroscopy (SERS) sensor, surface plasmon resonance (SPR) sensor, drug delivery, biomedical imaging and cancer therapeutics among others.
Reducing symmetry of Au nanoshells geometry shows interesting properties. It is possible to excite different plasmon modes in these particles when compared to standard particles. These particles show angle dependent plasmon resonance. This unique property may lead to a new class of optically active nanoparticles that can be manipulated by applied static or frequency dependent electric, magnetic, or optical fields. The particles enhance the electric field intensity coming out of the particles when compared to fully covered particles, i.e. particles whose symmetry has not been reduced.
Several groups have developed and demonstrated reduced-symmetrical nanoshells such as nano half-shells, nanocups, nanomoons and nanoeggs for SERS applications. Reduced-symmetrical nanoshells have been prepared before in various ways including electron-beam evaporation (EBE) and electroless plating. By these methods, the reduced-symmetrical structures of nanoshells, such as nano aperture or nanotip, are usually oriented randomly or with their aperture downward, which obviously limits the molecular binding to the electric field enhanced regions in SERS applications. The Raman enhancement factors differ from place to place on a substrate because of the random orientation of reduced-symmetrical structures.
In “Observation of plasmonic dipolar anti-bonding mode in silver nanoring structures”, Nanotechnology 20 (2009) 465203, Ye et al. explore plasmonic properties of silver and gold nanoring structures. It has been shown that reasonable SERS enhancement factors at near-infrared wavelengths can be achieved. Nevertheless, for a significant number of applications, the enhancement factor still is not high enough. Furthermore, for applications that do not involve single molecule measurements but aim at concentration determination, the enhancement factor integrated over the entire substrate is more relevant than the maximum enhancement factor. For the nanorings, the positions where high electric fields are reached are limited to the top edges of the nanorings, thus limiting the total interaction volume between the radiation and the molecules and thus limiting the integrated enhancement factor. There is still need for efficient Surface Enhanced Raman Scattering based sensing substrates.